GaN, AlN, InN, which are nitride semiconductors, or materials made of mixed crystals thereof, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. Among these, as high output electronic devices, technologies are developed in relation to Field Effect Transistors (FET), more particularly, High Electron Mobility Transistors (HEMT) (see, for example, Japanese Laid-Open Patent Publication No. 2002-359256). A HEMT using such a nitride semiconductor is used for high output/high efficiency amplifiers and high power switching devices.
Among nitride semiconductors, GaN has a polarity in a (0001) direction parallel to a c-axis (wurtzite form), and therefore, when a hetero structure of AlGaN/GaN is formed, piezo polarization is excited by lattice distortion of both AlGaN and GaN. Accordingly, in an AlGaN layer near the interface, high concentrations of Two-Dimensional Electron Gas (2DEG) are generated. Thus, GaN and materials including GaN are promising as materials of high frequency/electronic devices.
An example of HEMT using such a nitride semiconductor is formed by forming an AlN protection layer and an AlGaN buffer layer on a substrate made of, for example, silicon (Si), and forming a GaN electron transit layer and an AlGaN electron supply layer on the AlGaN buffer layer. However, when these semiconductor layers are caused to grow epitaxially on a silicon substrate, distortions are caused in the semiconductor layer due to differences in lattice constants between the semiconductor layers and the substrate and differences in thermal expansion coefficients between the semiconductor layers and the substrate. Accordingly, defects referred to as so-called cracks appear on the surface of the semiconductor layers. As illustrated in FIG. 1, these cracks 910 tend to be generated on the outer periphery of a substrate 920. These cracks 910 cause chipping and dust, and decrease the yield of the HEMT that is the semiconductor device to be manufactured.
In order to prevent the cracks 910 from being formed on the outer periphery of the substrate 920, there is disclosed a method of forming a protection film such as silicon nitride along the outer periphery of a substrate, and causing epitaxial growth by MOCVD (Metal Organic Chemical Vapor Deposition) (see, for example, Japanese Laid-Open Patent Publication No. 2009-256154).
Incidentally, as illustrated in FIG. 2, when a protection layer 930 such as silicon nitride is formed on the outer periphery of the substrate 920, a buffer layer 940 and a semiconductor layer 941 do not grow epitaxially on the protection layer 930, and therefore the buffer layer 940 and the semiconductor layer 941 are not formed on the protection layer 930. However, near the area where the protection layer 930 is formed, the source gas, which is supposed to be deposited in the area where the protection layer 930 is formed, diffuses from the protection layer 930. Therefore, there are cases where the buffer layer 940 and the semiconductor layer 941 grow abnormally. When abnormal growth is caused near the protection layer 930, the buffer layer 940 and the semiconductor layer 941 may become thick near the protection layer 930. Furthermore, the formed buffer layer 940 and the semiconductor layer 941 may have compositions and doping densities that are different from desired values. Furthermore, it is technically difficult to form the protection layer 930 only on the outer periphery of the substrate 920, and therefore costs may be increased.